Conventional blowers disclosed in Japanese Patent Unexamined Publication No. 2002-165477 and Japanese Patent Unexamined Publication No. H10-89674 will be described with reference to FIGS. 6 to 9.
FIG. 6 shows a circuit of conventional brushless DC motor 104 (hereinafter referred to as a motor 104). A direct-current voltage that is obtained by rectifying alternating-current voltage 101 is applied to motor 104 via inverter 105. The above circuit also has current detector 106, position sensor 107 of a rotor, and control device 110. Control device 110 has number of revolutions detecting unit 111, current detecting unit 112, air-volume calculating unit 113, and speed control unit 114 as shown.
FIG. 7 shows a circuit of speed control unit 114. Air-volume calculating unit 113 calculates the target number of revolutions of motor 104 for attaining a target air volume. Speed control unit 114 generates a PWM signal that has an output voltage for attaining a target number of revolutions. Inverter 105 performs PWM driving of motor 104.
When motor 104 is driven with a constant voltage, as shown in FIG. 8, air volume changes greatly with little change in number of revolutions. Accordingly, if a pressure loss changes slightly under the influence of external wind pressure, etc., the air volume will change greatly. The present invention provides a blower that realizes constant air volume control even when a pressure loss changes.